If you need to cancel or modify your registration, you will be able to do so any time before August 23 at 11:59pm MT. You will receive a full refund with the exception of a A$10 processing fee per ticket.
At the bottom of your Order Confirmation email you will see a link to view or modify your order. From that page you can also cancel individual tickets. Individual tickets may also be cancelled via the link at the bottom of your Ticket Confirmation email.
If your credit card or bank information has changed since you purchased your ticket, please contact your credit card company or bank prior to processing your refund. They should be able to make sure those funds get returned to you properly. If you have additional questions feel free to email firstname.lastname@example.org.